I have a 25 gl tank, here's what I do regarding water changes. I change 3-4 gallons a week.
Pre-mix water and salt a week in advance in a seperate 10 gallon tank or other clean container. You'll need to put in an airstone, powerhead, and heater as well.
Why?
Pre-mixing your water in advance causes less stress on your tank inhabitants.
It allows for the water to circulate/airate and the salt to mix completely, and the heater heats the water to the same temp as your main tank. Also, you can adjust the PH to the same PH as your main tank.
Sounds like a lot of work I know, but if you get into the habit early on, it's not so bad.
Oh yeah, try to use RO/DI wtare if possible...your fish will appreciate it.